The soil yeast Trichosporon cutaneum catabolizes the amino acids tyrosine and tryptophan by separate reaction sequences that converge upon beta-ketoadipic acid. Regulation mechanisms are different from those operating in prokaryotes; thus, salicylic acid derepresses the whole of the tryptophan-degrading sequence of enzymes. Two of these enzymes have been investigated: 2,3-dihydroxybenzoate decarboxylase, which apparently requires no coenzyme, has been purified and characterized; and an NADPH-dependent hydroxylase for anthranilate (liberating ammonia) was shown to incorporate two oxygen atoms, one from water and the other from dioxygen. Enzymes and catabolites involved when some strains of Escherichia coli grow aerobically at the expense of the products of fecal, anaerobic degradation of phenylalanine and tyrosine by other organisms have been characterized. Degradation of methoxylated derivatives of phenylalanine catabolites is accomplished by various species of Pseudomonas and Achromobacter; methanol is released, but is not utilized by the bacteria studied. These methoxylated aromatic acids arise in nature when fungi degrade lignin, and it is probable that the reactions we have investigated provide the carbon source for methanol-utilizing microorganisms which are widely distributed in soil.